Once they had taken the seal of the finger off, other cursed spirits that the finger had drawn in attack the club members. As Yuji Itadori and Megumi Fushiguro came in to save the club members, Yuji eats the finger in order to fight off the cursed spirits.

He is simply a human who existed long ago in the past and was a powerful sorcerer. Yet, Sukuna, in his true form, has four arms instead of two and an extra mouth. This has not been explained in the series at all, however, there are some popular ideas out there that explain why he has two extra arms and an extra mouth.

Ryomen Sukuna's fingers are severed mummified fingers that originally belonged to Sukuna. There are a total of twenty of them, each containing the soul and fragmented power of Sukuna even after a thousand years. Each of the fingers possesses cursed energy that is equivalent to a special grade cursed spirit.

Okay first off Sukuna is not a cursed spirit. He was a sorcerer in the heian era who was then turned into a cursed object. He is a REINCARNATED SORCERER. While we don't know the exact reason why he is so strong, it is clear from the manga till now that he is very much a person who appreciates Jujutsu.